Permanent magnets



United States Patent Ofilice 3,314,828 Patented Apr. 18, 1967 3,314,828 PERMANENT MAGNETS John Harrison, Sheifield, England, assignor to Swift Levick & Sons Limited No Drawing. Filed Jan. 22, 1964, Ser. No. 339,320 4 Claims. (Cl. 14831.57)

This invention relates to permanent magnets of the Fe-Al-Ni-Co-Cu type and is essentially concerned with the high-energy forms of this type based on a columnar crystal structure. The general composition by weight, of alloys believed to be capable of yielding a beneficial columnar structure, can be taken as regards these main constituents named, as embraced by:

Percent Al 5 to 11 Ni 7 to 25 Co 20 to 40 Cu 0 to 10 Fe Balance The balance however usually comprises, additionally to Fe, minor amounts of other constituents present as unavoidable impurities, not having any significant effect on the magnetic properties. More importantly, it may also comprise constituents deliberately introduced in significant (but not necessarily substantial) amount for the improvement of one or more of the ultimate magnetic properties, or the adjustment of the magnetic characteristics as delineated by the demagnetisation curve.

Thus Ti has long been known to lead to an increase in coercivity. Nb and Ta (usually available in association, and therefore generally considered in terms of the amount of Nb actually present) may replace part of the Ti present with that object, the Nb and Ta conferring beneficial heat treatment characteristics on the high coercivity alloys. Nb and/ or Ta in the absence of Ti will not impart coercivity of the high order obtainable with Ti. However, Nb (and/or Ta) are conducive to the formation of a columnar crystal structure, with which the greatest benefits of anisotropy are associated when the crystallisation is in a preferred direction of magnetisation.

Yet again, the balance may include constituents similarly present by deliberation for the improvement of the mechanical properties of the magnet alloys, notably S for amelioration of the brittleness that is a characteristic of these alloys, to an extreme degree with some compositions. However, S diifers from some other minor constituents the presence of which below a restricted individual or collective maximum can be accepted as impurity, if it is not brought about by deliberate addition, e.g., P, Zr, Cr, Mn, C, and Si, which can be designated as benign because of their lack of effect on the magnetic properties, even though some adjustment of the heat treatment may be necessary to allow for, or to take advantage of, their inclusion in the alloy, whereas such minor constituents are taken into solution, S is not, and its presence above the trace level is not without effect on the magnetic properties. As the S content increases above that level, so do the magnetic properties ordinarily decrease, this being believed to be due to the formation of non-magnetic sulphides in the alloy and a consequent dilution effect.

It is, however, by no means the case that all combinations of such different constituents, minor or otherwise, each tending to some improvement in the final magnets, can contribute as a useful total to the balance, consisting predominently of Fe, of a particular composition of the main constituents: incompatibility of one sort or another is known to arise. In particular, Ti, in the ordinary way is definitely not conducive to the formation of a columnar crystal structure.

Because of these considerations, energy product (BH) whilst the highest of commercially manufactured anisotropic materials has been brought into the range 7 to 8X10 gauss-oersteds by the columnar crystal technique, the coercivity H has been found to be in the range 700 to 800 oersteds. That coercivity is much below the 1000 to 1200 oersteds or more obtainable, at the expense of much lower energy (5x10 gauss-oersteds) and remanence, by an appropriate amount of Ti, Nb, or Ta, in magnets to which the columnar crystal technique is inapplicable.

High coercivity and high energy have up to the moment only been obtainable simultaneously by the application of a special technique on special materials, not applicable to the production of commercial magnets.

Thus (see Phillips Research Reports 11, 1956, pages 489 to 490), rods of pure alloy Percent Ni 15 Al 7 Co 34 Cu 4 Fe 35 Ti 5 prepared by melting pure metals in pure argon were used for the preparation by a crystal-pulling technique of single crystals having an axis nearly parallel to the direction of pulling, with the following properties:

(BH) gauss-oersteds 11x10 mm gauss 11,800 H oersteds 1315 In the report just referred to reference is made to the decrease in (BH) resulting from the use of Ti to attain high H In Cobalt, December 1961, Wright and Thomas conclude that it is the combination of Ti and Al together, in various compositions falling within what is usual for magnet alloys of the type now in question, that leads to fine equi-axed crystals in magnets subjected to the chilling technique as employed for the production of columnar crystal magnets.

Contrary to the evidence of existing practice and belief, the present invention is based on the discovery that the apparent incompatibility of Ti as regards simultaneous attainment of high energy and high coercivity can be overcome, and overcome without significant departure from the ordinary commercial method of producing permanent magnets.

The discovery is that Se, alone or together with S (which is ordinary considered as improving only the mechanical properties), present in significant but limited amount in conjunction with Ti, or with both Ti and Nb (Cb), enables energy and coercivity to be simultaneously achieved to values not previously attainable in commercial magnets.

According to the present invention, and anisotropic columnar crystal magnet has the composition by weight:

At least $152 g roiip compfisi ng s 1555 Se 0.05 to 2 Fe and impurities Balance r3 3' Preferably, the composition is:

' Percent Al 6 to 7.5 Ni 13 to 18 Co 20 to 40 Cu to 6 Ti 1 to 6 Nb (Cb) 0 to 3 Si Up to 0.5 At least one of the group comprising S and Se M 0.1 to 0.75 Fe and impurities Balance S is preferably within the range 0.1% to 0.4%. Se is preferably within the range 0.2% to 0.75%. But 8 and Se may be used together in equivalent atomic percentages to make up any desired total of the two inside the broad range 0.05% to 2% or the preferred range 0.1% to 0.75%.

As shown by specific examples given below, allows falling within the preferred composition are found to have magnetic properties of the following order:

B 10,000 to 13,000 gauss. H mainly 1000 to 1350 oersteds. (BH) 5.1 to 10.5 10 gauss-oersteds.

when cast by the usual chilling technique and subjected to magnetic cooling and tempering as ordinarily accorded to the type of alloys used for columnar crystal magnets.

It is, however, an advantage of the invention to enable the general advance in coercivity attributable to the presence of Ti in mag-nets with equi-axed crystals to be utilised in magnets with columnar crystals even in the case where the combined values of coercivity and energy do not reach those attainable with S or Se, or with Se and S together, in the preferred range of 0.1% to 0.75%: hence the inclusion within the scope of the invention of the somewhat wider range of 0.05 to 2.0%.

It is believed that the S or Se, alone or together, forms a barrier of immiscible liquid sulphides between certain crystal and nuclei and the melt in the case of magnet alloys containing Ti, to overcome the tendency of the nuclei to promote rapid random crystallisation throughout the cooling liquid, and thus to permit crystallisation to proceed by columnar growth.

The S may be conveniently introduced as ferrous sulphide and the Se as ferrous selenide (ferro-selenium). The introduction is best effected with the main ingredients of the initial charge to be melted, rather than-as is done with the Al-at or towards the end of the melting operation.

The examples of which particulars are given in Tables 1 and 2 show the magnetic properties obtained by the addition of S in columnar crystal magnets with various compositions, most of them including both Ti and Nb (Cb), but one having no Nb (Cb). In some instances, the magnetic properties obtained in equi-axed magnets of like or comparable composition are given for comparison. Examples 1 and 6 of Tables 1 and 2 are given for comparison only, the S being below and above respectively the preferred range when S is used in the absence of Se.

In all cases, the magnets of the examples were subjected after casting (with of course chills in the case of the columnar crystal examples) either to heat treatment appropriate in the one case to equi-axed magnets (designated as EE) or in the other to columnar crystal magnets (designated as CC).

Except for Example 7, the EB was:

initial heat treatment Solution temperature 1250" C. Magnetic cooling To 600 C. in 14 mins.

In Example 7, this treatment was:

4. Except for Example 8, the CC initial heat treatment was:

To 1000" C. in 2 mins. To 600 C. in 10 mins.

Solution temperature Fast cool (air blast) Magnetic cooling In Example 8, this treatment was:

To 1000 C. in 10 secs. To 600 C. in 13 mins.

Solution temperature Fast cool (air blast) Magnetic cooling In all cases, tempering was:

48 hours at 590 C., followed by 48 hours at 560 C.

Examples eing Fe and impurities (Ex. 7 nominal)] Ex. A1 Ni 1 Co Cu Tl Nb (Cb) S TABLE 2.-MAGNETIC PROPERTIES (BHLML. Em... gauss He, oersteds gaussoersteds 8, 850 1,142 4.0 X10 10, 220 1, 248 5. asxio ,630 1,280 6- X10 10, 520 1, 265 6 45 10 10, 200 1, 182 5 35x10 9, 970 1, 077 4 58X10 9, 000 1, 070 31 75 10 11,000 1,090 6.45X10 The examples of which particulars are given in Tables 3 and 4 embrace the addition of S within the stipulated range to an extended range of alloy compositions, including Nb-containing alloys within the composition:

and also Nb-free alloys lowing compositions:

within one or other of the fol- TABLE 3 Ex. Al Ni C0 1 C11 T1 l Nb (Cb) S References Cited by the Examiner 3,144,324 8/ 1964 131th? 75-422 UNITED STATES PATENTS 3,175,901 3/1965 lesmont 75-124 2,295,082 9/ 1942 1011313 75-424 X HYLAND BIZOT, Primary Examiner. 2,395,285 2/1946 McKlbben 75124 5 DAVID L RECK, Examiner 2,797,161 6/1957 Ireland 75-124 2,837,452 6/1958 De V05 75-424 P. WEINSTEIN, Assistant Examiner. 

1. AN ANISOTROPIC COLUMNAR CRYSTAL MAGNET CONSITING ESSENTIALY OF: 